Having passed through the vacuum of outer space, the light from the celestial body has to pass through the atmosphere before it is observed using a sextant. The atmosphere tends to bend the path of the light as it passes through, so that the celestial body appears higher in the sky than it would be if there was no atmosphere.

A calculated Altitude assumes the earth has no atmosphere, so the sextant Altitude has to be corrected for Refraction so that it can be compared to the calculated Altitude. Refraction tables are always included in Almanacs used for celestial navigation.